In this article I will explain with an example, how to implement Server Side validation of RadioButton in ASP.Net Core Razor Pages.
The Server Side validation will be performed for RadioButton using Model and Data Annotations in ASP.Net Core Razor Pages.
The following Model class consists of one property Gender. The property is decorated with the following Data Annotation attributes for performing validations.
1. Required Data Annotation attribute.
The Data Annotations attributes can be used with the Entity Data Model (EDM), LINQ to SQL, and other data models.
The Required Data Annotation has been specified with a property Error Message with a string value. As the name suggests, this string value will be displayed to the user when the validation fails.
using System.ComponentModel.DataAnnotations;
namespace RadioButton_Validation_ServerSide_Razor_Core.Models
    public class PersonModel
        [Required(ErrorMessage = "Gender is required.")]
        public string Gender { get; set; }
Razor PageModel (Code-Behind)
The PageModel consists of following two Handler methods.
Handler method for handling GET operation
This Handler method handles the GET calls, for this particular example it is not required and hence left empty.
Handler method for handling POST operation
This Handler method handles the POST operation and when the form is submitted, the object of the PersonModel class is sent to this method.
The state of the submitted Model is checked using ModelState.IsValid property and if the Model is valid then the value of the Gender property is set in a ViewData object.
Note: ModelState.IsValid property is an inbuilt property which verifies two things:
1. Whether the Form values are bound to the Model.
2. All the validations specified inside Model class using Data annotations have been passed.
public class IndexModel : PageModel
    public string Gender { get; set; }
    public void OnGet()
    public void OnPostSubmit(PersonModel model)
        if (ModelState.IsValid)
            this.Gender = model.Gender;
            ViewData["Gender"] = this.Gender;
Razor Page (HTML)
The Form
The HTML of Razor Page consists of an HTML Form which has been created using following attribute.
method – It specifies the Form Method i.e. GET or POST. In this case it will be set to POST.
The Form consists of HTML Input RadioButtons, HTML Label for displaying the text for each RadioButtons, a SPAN and a Submit Button.
The Input RadioButtons has been assigned with asp-for Tag Helper attribute and its value is set with the Gender property of the IndexModel class. Thus, this signifies that the validation will be performed for the Gender property.
The SPAN has been assigned with asp-validation-for Tag Helper attribute and here also its value is set with the Gender property of the IndexModel class which signifies that it is used for displaying the Validation message for the Gender property.
The Submit Button has been set with the POST Handler method using the asp-page-handler attribute.
Note: In the Razor PageModel, the Handler method name is OnPostSubmit but here it will be specified as Submit when calling from the Razor HTML Page.
When the Submit button is clicked, the Form gets submitted and the Gender value is sent to the POST Handler method.
Displaying submitted value
Below the Form, the ModelState.IsValid property is checked and if the Model is valid, then the value of the ViewData object is displayed using Razor syntax.
@addTagHelper*, Microsoft.AspNetCore.Mvc.TagHelpers
@model RadioButton_Validation_ServerSide_Razor_Core.Pages.IndexModel
    Layout = null;
<!DOCTYPE html>
    <meta name="viewport" content="width=device-width" />
    <style type="text/css">
        body { font-family: Arial; font-size: 10pt; }
        .error { color: red; }
    <form method="post">
                    <input type="radio" name="Gender" asp-for="Gender" value="M" />
                    <input type="radio" name="Gender" asp-for="Gender" value="F" />
                <td><span asp-validation-for="Gender"class="error"></span></td>
                <td><input type="submit" value="Submit" asp-page-handler="Submit" /></td>
        <hr />
        @if (ViewData.ModelState.IsValid)
ASP.Net Core Razor Pages: Server Side RadioButton validation